I've finished my first programming book, where do i go from here?

I've finished my first programming book, where do i go from here?

I have recently finished my first programming book "Programming in the key
of C#" by Charles Petzold. I know this book my be outdated (2004) but
being a recent broke high school grad it was a steal for $5 at my local
used book store. After reading this I feel like I have a decent grasp of
the concepts of OOP and C# specifically, but I am kind of lost on where to
go from here. I have been working through the project euler problems and
have been having a decent amount of success (I have finished the first 15)
and I have been writing basic programs from scratch using the information
I learned from this book, but I feel like I need something new to study to
expand my knowledge past the basics. Any recommendations on where to go
from here would be appreciated.